This invention relates to the subject of electronic emission from the surface of a metal; and more particularly to, methods for making low work function electron-beam cathodes which emit electrons in a cone-shaped pattern with a substantially uniform electron density throughout the cone.
In the prior art, one low work function electron-beam cathode has been described in U.S. Pat. No. 3,814,975 issued June 4, 1974 to Wolfe et al. Basically, that cathode consists of a single crystal tungsten needle, having a ring of zirconium and a hairpin-shaped filament attached to it.
During operation, the cathode is heated via the filament, which causes the zirconium to migrate from the ring over the surface of the tungsten needle to its tip. There, the zirconium atoms together with oxygen atoms in the residual gas of a vacuum that surrounds the needle cause the needle's work function to be reduced from 4.5 ev (for pure tungsten) to 2.7 ev.
One undesirable characteristic, however, of that cathode is that its work function is highly dependent on the needle's temperature and the pressure of the surrounding residual gas. In particular, for any given specific temperature, there is only one optimum operating pressure at which the beam current along the needle's axis is a maximum.
Still another undesirable characteristic of that cathode is that the density of electrons which are emitted from its tip is non-uniform. That is, for any given operating temperature and pressure, the electron density alternatively varies from a relatively high value, to a relatively low value, then to another high value, then to another low value, etc. as the angle of emission increases from the needle's axis. These variations in electron density can be "seen" experimently, by bombarding a phosphorous target with electrons from the cathode, and observing the visible light pattern which results. That pattern is in the form of a plurality of concentric rings.
Another cathode, which is a substantial improvement over the above '975 cathode, is described in co-pending United States patent application entitled "Low Work Function Cathode" by Wolfe et al filed Apr. 30, 1980 and assigned Ser. No. 145,042. Basically, this cathode is formed by a novel process which makes zirconium soluble in solid tungsten and produces a single crystal cathode having host atoms of tungsten and dopant atoms of zirconium and oxygen. The cathode which is produced by this method has a work function of 2.7 ev regardless of the operating temperature and pressure of the surrounding gas, so long as that gas is essentially oxygen free.
A problem which remains however, is that these cathodes also emit electrons with a non-uniform density as a function of angle from the needle's axis. The present invention address that problem and its solution.
Accordingly, a primary object of this invention is to provide an improved electron-beam cathode.
Another object is to provide a low work function electron-beam cathode which emits electrons in a cone-shaped pattern whose electron density is substantially uniform throughout the cone.
Still another object is to provide a method of making an electron-beam cathode which meets the above objectives.